1. Technical Field
The present invention relates to clutch mechanisms, and more particularly, to a clutch mechanism with overload protection, which is applicable to motor-driven devices requiring overload protection, such as actuators.
2. Description of Related Art
With the progress of automation, linear actuators have become a vital component in automated machines and are extensively used in electric beds, lift chairs, electric tables, lift platforms, power windows and so on. A linear actuator is conventionally composed of a housing, a motor, a transmission mechanism and a push rod. The motor transfers a rotational motive force to the push rod through the transmission mechanism. The push rod comprises a screw and an extension tube. The screw has a screw shaft and a nut, wherein the nut converts the rotational motive force from the screw shaft into a linear thrust that drives the extension tube attached to the nut so that the extension tube drives a load to move.
In the conventional linear actuator, for protecting the push rod and the load from receiving excessive torque and getting broken, a clutch mechanism is provided and serves to disengage the push rod from the motor when the push rod is jammed or overloaded. Traditionally, the clutch mechanism is realized by an electromagnetic clutch, which has to work with an additional control box incorporating an overload-detection-and-protection circuit and an external power source, so that the power transmission between the motor and the push rod can be controlled with electromagnetic induction. However, after a long term of use, the electromagnetic clutch tends to have its coils worn and thus is not durable. Furthermore, the electromagnetic clutch only provides the actuator with indirect overload protection that completely depends on the external power source. Moreover, since the electromagnetic clutch is designed to perform the functional disengagement when the power is off, it is unsuitable for a system that normally needs engagement, such as a hospital bed, where once the power is off, the weak self-locking force between components thereof may cause any of the components to collapse and hurt the user and people around. In addition, the control box makes the actuator extra bulky, being adverse to a compact layout of the automated machine using such an electromagnetic clutch.